OLED (Organic Light-Emitting Diode) display device is a novel flat panel display device. In comparison to Liquid Crystal Display (referred to as LCD), since OLED is able to emit light by itself without any backlight module, the OLED has lower power consumption than the LCD.
In an OLED display device, OLED pixel structures are generally arranged in a matrix manner. The driving modes of the OLED display device may be classified into a Passive Matrix Organic Light Emission Display (PMOLED) driving mode and an Active Matrix Organic Light Emission Display (AMOLED) driving mode. In comparison to the PMOLED pixel structure, the AMOLED pixel structure has the advantages of high resolution, high precision and the like.
At present, during the manufacturing process of an OLED display device, generally, FMMs (Fine Metal Masks) are used as evaporation masks for forming a light-emitting layer through evaporation by using light-emitting material. An opening region of an evaporation mask corresponds to a deposition region of the light-emitting material in each of sub-pixels. The smaller the opening region is, the smaller the sub-pixel is, and the smaller pixel may be obtained accordingly. Thus, the PPI (Pixels Per Inch) is higher, and the resolution and precision are higher. A fact is that the opening region cannot be fabricated as too small because the FMM mask has a certain thickness. Another fact is that the product yield is very low when evaporation masks having a smaller thickness are used to perform evaporation.
Meanwhile, present evaporation masks used for the vacuum coating of AMOLED pixel structures are mainly made of INVAR (invar alloys, also referred to as Invar, one of important structural materials) metal material. The INVAR metal material is difficult to obtain, the cost of the INVAR metal material is very high, and the width of a Rib region (i.e., a shielding region) cannot become smaller due to too large thickness of the evaporation masks, so that the PPI of AMOLED products cannot be improved substantially.
FIG. 1 shows a plan view (the upper diagram in FIG. 1) and a cross-sectional view (the lower diagram in FIG. 1, taken along the direction A-A in the upper diagram) of an evaporation mask in the prior art. In FIG. 1, the blank region is a shielding region 6 (corresponding to a non-evaporation region, i.e., the blank rectangle in the upper diagram of FIG. 1, and the trapezoid cross-section taking Rib Width and Slit Width as bottom edges in the lower diagram of FIG. 1) in the evaporation mask, and the cross-hatched region is an opening region 7 (corresponding to an evaporation region, i.e., Slit Space in FIG. 1, referred to as SP) in the evaporation mask. When the evaporation mask shown in FIG. 1 is used to shield a coating (e.g., to shield the shielding region 6 corresponding to sub-pixels G and B), a coating for forming any two of the sub-pixels in a pixel unit (comprising three sub-pixels, i.e., R, G and B) is shielded in one evaporation process. The opening region 7 of the evaporation mask in the plan view has greater influences on the performance of the evaporation than an comprised angle θ (i.e., gradient) of the evaporation mask in a thickness direction in the cross-sectional view. The specific reasons are as follows.
The width Slit Width of the shielding region 6 of the evaporation mask is:Slit Width=b+2*a=b+2*d*c tan(θ)  (1).
In the equation (1), the comprised angle θ of the evaporation mask in a thickness direction is generally ranged from 40° to 60°.
The width (i.e., the sum of widths of three sub-pixels) of the pixel pitch is:Slit Width+SP≅1.5*Slit Width=1.5*(b+2*d*c tan(θ))  (2).
In the equation (2), b is mainly limited by precision of etching process, so the pixel pitch cannot be small enough mainly due to the thickness d of the evaporation mask.
At present, the thickness of the evaporation mask made of INVAR metal material is usually 40 μm, 30 μm at minimum. At present, even though the thinnest evaporation mask is employed, the PPI cannot be improved substantially. Moreover, by using the thinnest evaporation mask, the width cannot be remained at the original level, and the product yield will be reduced instead.